Ester preparation by oxidation of hydrocarbons



July 31, 1962 R. E. PENNINGTON ETAL 3,047,612

ESTER PREPARATION BY OXIDATION OF' HYDROCARBONS Filed NovY '7. 1958 2 Sheets-Sheet l ATTORNEY.

July 31, 1962 R. E. PENNINGTON ETAL. 3,047,612

ESTER PREPARATION BY OXIDATION oF HYDRocARBoNs Filed Nov. '7, 1958 2 Sheets-Sheet 2 ATTORNEY.

United States Patent Ofiice 3,047,431@ ESTER PREPARATEN BY @XDATHN @E HYDRGCARENS Robert E. Pennington and Louis T. Yule, Baytown, and Charles F. Snyder, Houston, Tex., assignors, by rncsne assignments, to Esso Research and Engineering Cornpany, Elizabeth, N3., a corporation ot Belaware Filed Nov. '7, 1958, Ser. No. '772,500 6 Claims. (Cl. 26d- 475) This invention is directed to a method for the preparation of methyl esters of polycarboxylic aromatic acids. More particularly, this invention is directed to a process for the substantially simultaneous production and esteriication of aromatic polycarboxylic acids.

The preparation of methyl esters of polycarboxylic aromatic acids presents many difficulties, not only With respect to the preparation of the polycarboxylic aromatic acids from alkyl aromatic hydrocarbons 4but also with respect to the esterification of such acids. Thus, when the acids are to 'be prepared by the oxidation of polyalkyl aromatic hydrocarbons with molecular oxygen, it is found that the oxidation reaction is carried to completion only with great ditculty and that serious problems are encountered with respect to by-product formation and product purification. For example, the air oxidation of a first alkyl group of a polyalkyl benzene results in the formation of a refractory alkylated benzoic acid which is highly resistant to further oxidation. As a consequence, the air oxidation of a second alkyl group is accomplished only with great difiiculty. Moreover, the acids, once prepared, are only sparingly soluble in methanol and are therefore difficult to esterify.

lt has now been surprisingly discovered that methyl esters of polycarboxylic aromatic acids may be directly prepared from polyalkyl aromatic hydrocarbons with ease by bringing oxygen into contact with a mixture of a liquid polyallyl aromatic hydrocarbon under liquid phase oxidation conditions with a molar excess of a liquid methyl alcohol in the presence of a catalytic amount of a heavy metal salt. Thus, a product is obtainable wherein at least one (and preferably all) of the alkyl groups of the polyalkyl aromatic feed stock are converted to methyl ester groups.

More particularly, the present invention comprises a liquid phase process for the oxidation of polyalliyl aromatic hydrocarbons with molecular oxygen in the continual presence of an excess of liquefied methanol and the substantial absence of volatilized methanol, the process being catalyzed with a catalytically eilective amount of a polyvalent metal oxidation catalyst. when this is done, oxidation of the methanol is surprisingly minimized and esterification of carboxyl groups is surprisingly enhanced. In addition, the presence of surprisingly large amounts of water in the liquid phase does not substantially adversely affect the process.

The starting material for the present invention is aV substantially wholly aromatic hydrocarbon feed stock containing at least one aromatic hydrocarbon ring substituted with a plurality of C1 to C, alkyl groups. Thus, the feed stock may consist of a single purified polyalkyl aromatic hydrocarbon or may comprise a mixture of a plurality of polyalkyl aromatic hydrocarbons or even a mixture of one or more polyalkyl aromatic hydrocarbons with an unsubstituted aromatic hydrocarbon or a monoalkyl aromatic hydrocarbon, or both. By way of example, the starting compounds, which may be used alone or in admixture include polymethyl benzenes (o, m, and p-xylene; pseudocumene; mesitylene; hemimellitene; durene; isodurene; prehnitene; pentarnethyl benzene, and hexamethyl benzene), the corresponding polyethyl, polyn-propyl., polyisopropyl, polybutyl, and polyisobutyl benzenes, and the corresponding polybenzyls such as polyalliyl substituted diand tri-benzyls. Representative examples of such additional feed stocks include 0, m-, or p-cymene; 1,2-, 1,3-, or 1,4-diethyl benzene; -methyl- 3-butyl benzene; 1-methyl-4-tert.-butyl benzene; 1,2-, 1,3-, or 1,4-disopropyl benzene; 1,2,4- or 1,3,5-triethyl benzene; l,2,3,4- or l,2,4,5-tetraethyl benzene, etc. As indicated, compounds as above described may be used alone and in admixture with each other or in adinixture with other aromatic hydrocarbons such as benzene, monoalkyl benzenes (eg, toluene, ethyl benzene, isopropyl benzene), or both.

The amount of methanol that is required is-dependent upon a number of factors. In general, in excess of one mol of methanol is required per mol equivalentof alkyl group to be oxidized. More preferably, about 1.5 to about 3 mols of methanol should be used per mol equivalent of alkyl groups to be oxidized. Thus, if it is desired to convert a xyiene to the corresponding methyl diester, there may be used, for example, a total of from about 3 to about 6 mols ot methanol per mol of xylene (i.e., 1.5 to about 3 mols of methanol per mol equivalent of methyl group to be oxidized).

The manner in which the methanol is added to and `maintained in the reaction mixture may vary, provided only that the methanol that is introduced is maintained substantially (preferentially) in the liquid phase after its introduction. lt is not practical to maintain all of the methanol in liquid phase. For example, if dry air is used as the source of molecular oxygen, there will be a tendency for the methanol in liquid phase to saturate the air with methanol vapor. In addition, the gases that flow through the liquid reaction mixture will tend to entrain droplets of the liquid components including hydrocarbons, methanol, and water. However, in accordance with the present invention, the process is conducted so as to maximize the amount of methanol that is maintained in the liquid phase. Thus, by way of definition, the methanol is preferentially maintained in the liquid phase when methanol is lost from the liquid phase to the vapor phase only by saturation of the charged air and by entrainment. All of the required amount of methanol may be added with the fresh feed. Preferably, however, only a portion of the alcohol is added with the fresh feed and the remaining portion is added periodically or continually during the course of the reaction. Accordingly, in batch operations all of the methanol may be added at the beginning of the reaction or may be periodically or continually added during the course of the reaction. In continuous operations, a single stage may be employed with all of the methanolbeing mixed with the fresh feed. A plurality of stages may be employed with fresh methanol being added to each stage.

The catalysts to be utilized in accordance with the present invention are salts of polyvalent heavy metals such as the naphthenates, acetates, bromides, chlorides, etc. of cobalt, manganese, etc. Mixtures of two or more such heavy metal salts may be utilized if desired. At least a catalytically effective amount of catalyst should be employed. Thus, an amount of catalyst compound may be used which is sufficient to maintain about 0.05 to about 0.8 weight percent of catalyst in soluble catalytically effective form in the liquid reaction mixture. Larger amounts of catalyst (eg, up to about l0 percent) may be employed, if desired, but this is not absolutely necessary. rThe amount of catalyst to be used, as set forth above, is calculated with respect to the weight of the heavy metal of the catalyst compound and the polyalkyl aromatic hydrocarbon feed stock.

Promoters for the oxidation reactions, the esterification reactions, or both, may be and preferably are employed in catalytically effective amounts. Thus, soluble Patented duly 3l., 1962 t bromine salts such as cobalt bromide, ammonium bromide, manganese bromide, etc., acids such as hydrobromic acid, hydrochloric acid, or sulfuric acid may be employed in catalytically eective amounts. Normally, from about' 0.5 to 2 volume percent of promote-r is employed, based on the total charge. A preferred promoter is hydrogen bromide. This compound has several advantages. Thus, it is characterized by high promotional activity, both with respect to the oxidation reactions involving the polyalkyl aromatic feed stock and the esterication reactions involving the methanol and aromatic acids produced by oxidation `of the alkyl aromatic feed stock but does not promote oxidative degradation of the methanol.

The reaction conditions to be employed are, in general, the reaction conditions that are normally employed in the liquid phase air oxidation of polyalkyl aromatic hydrocarbons in the substantial absence of liquid methanol. However, it has been discovered, in accordance with the present invention, that when such conditions are employed in the presence of liquid methanol, both oxidation and esterification reactions are promoted while oxidative decomposition of the liquid methanol to liquid nonaromatics is largely inhibited and oxidation of the methanol to gaseous products such as carbon dioxide and water is almost completely suppressed. l

The reaction conditions to be employed should include a temperature within the range of about 140 to about 250 C. The pressure employed should be sufficient to maintain `the methanol substantially exclusively in liquid phase. Accordingly, for temperatures within the range of about 140 to about 250 C., the pressure employed may be, correspondingly, within the range of about 200 to 1000` p.s.i.g.

When this is done, only a minimized amount of the methanol will be present in the reactor in vapor phase, for the reasons set forth above. This is desirable in that `side reactions of the alcohol `are thereby minimized. lf this is not done, substantial degradation of the methanol will occur.

The oxidizing medium to be employed is oxygen, including pure oxygen, air, etc. Although the oxygen charge rate is not particularly critical, for convenience of reaction it is generally preferable to employ an oxygen charge rate such that, with the gas-liquid contacting available, an oxygen utilization of 50 percent or greater is accomplished. The oxygen charge rate should be corre-- lated with the rate of reaction that is obtainable. With poor gas-liquid contacting, the reaction rate will be slow and comparatively low oxygen flow rates should be employed. With good agitation, good gas-liquid contacting is obtained whereby substantially higher reaction rates can be achieved and correspondingly higher oxygen flow rates can be utilized.

Preferably, the oxygen charge rate will be balanced with oxygen consumption so that the tail gas will be substantially completely free from unreacted oxygen. When the source of molecular oxygen is air, it is desirable that the tail gas contain not more than about l5 volume percent of oxygen. Oxy-gen llow rates which will cause entrainment of excessive amounts of methanol should be avoided.

Surprisingly large amounts of water may be tolerated in conducting the process of the persent invention. However, when excessively large amounts of water are present, both esterication and oxidation rates are retarded. Of course, the water of reaction may be removed substantially contemporaneously with its evolution in order to maintain a substantially anhydrous condition within the reactor. However, this is not necessary to the practice of the present invention. Since the substantially complete removal of water from the reaction system without vaporization of the methanol is technically diiiicult, if not impossible, it is an advantageous feature of the present invention that substantial quantities of water (about 2 to 4 mols of water per mol of aromatic components) may be permitted to accumulate in the liquid phase without severely, adversely alfecting the economics of the process. The amount of water that is permitted to accumulate in the reaction mixture should not be so excessive as to unduly dilute the methanol. Thus, it is preferable that the liquid methanol comprise at least l5 weight percent of the liquid phase, and still more preferably from about 20 to about 501 weight percent.

It is preferable to employ a continuous process with byproduct purges as needed. As indicated, it is within the scope of the present invention to conduct the process in a single stage or in a plurality of stages. When a plurality of stages are employed, all of the fresh feed may be introduced into the first stage and the rst stage may be operated in a manner to at least partially oxidize a substantial portion of the fresh feed (e.g., 60 to 95 mol percent) and to esterify a substantial portion (e.g 60 to mol percent) of the carboxyl groups formed by the oxidation reaction. That is to say, in the first stage oxygen addition to the fresh feed is the important consideration and no attempt is made to drive the oxidation and esterification reactions to completion. The oxidation and esterication reactions are thereafter completed in subsequent stages.

By-product water of reaction may be permitted to accumulate in the liquid phase of each stage but is preferably removed from the discharge material from the stage before the material is charged to a subsequent stage. Thus, the total product from each stage may be freed from water and alcohol oxidation byproducts by distillation and various other means familiar to those skilled in the art.

The invention will be further illustrated with respect to the accompanying drawings wherein:

FIG. l is a schematic ow sheet illustrating one manner in which a continuous single stage process may be conducted in accordance with the present invention; and

FIG. 2 is a schematic ow sheet illustrating one manner in which the present invention may be practiced employing a plurality of stages.

SINGLE STAGE Pnocnss Turning now to FIG. 1, there is shown a reaction zone 200 such as a corrosion resistant (e.g., glass-lined or titanium) reactor provided with suitable agitating means such as an impeller 202.

Fresh methanol for the process is introduced by way of a charge line 204 leading to an incorporator 206. Recycle methanol to be obtained in a manner to be described may be added to the charge line 204 by either a recycle line 208, a recycle line 210, or both.

A feed stock consisting essentially of aromatic hydrocarbons and containing at least a predominant amount of a polyalkyl aromatic hydrocarbon is charged from a suitable source (not shown) by way of a charge line 2l2 to the incorporator 206. A compound of a catalytically eective heavy metal such as cobalt naphthenate, t0- gether with a suitable promoter such as hydrogen bromide, may be added to the charge line 2112 by way of the catalyst charge line 214 controlled by a valve 216. Recycle products obtained in a manner to be described may be added to the aromatics charge line 212 by way of a recycle line 218, a recycle line 220 controlled. by a valve 222, or both.

The thus-prepared reaction mixture is discharged from the incorporator 206 by way of a line 224 leading to a preheater 226 wherein the charge is heated. The heated charge is introduced into the reaction zone 200 by way of a charge line 22S. A suitable source of molecular oxygen such as air is introduced into the reactor 200 by way of oxygen charge line 230.

Within the reaction zone 200, the polyalkyl aromatic hydrocarbon feed stock is at least partially oxidized and a substatnial portion of the carboxyl groups that are formed as a result of the oxidation reactions are esteried with the methanol. As a consequence, there is formed in the reaction Zone 2li@ a liquid reaction ture containing unreacted feed stock components, unreacted methanol, aromatic feed stock oxidation products, esterication products, and liquid methanol degradation by-products.

Olii-gas will accumulate above the level of the liquid in the reaction Zone and will contain nitrogen (when the source of oxygen is air), unreacted oxygen, if any, minor amounts of carbon dioxide, or carbon monoxide, or both, and such amounts of the feed components and reaction products as are present in the gas. The off-gas is withdrawn from the reaction zone o by Way of a vent line 232 adjacent the top of the reaction zone leading to a cooler 234 wherein condensable components of the otigas are liqueed. The resultant mixture of liquids and gases is discharged from the cooler 234 by Way of a charge line 236 leading to a separating drum 238. The uncondensed gas is vented from the settling drum 238 by Way of a vent line 24o and discharged from the system.

Condensed components of the olf-gas will include unreacted methanol and methanol by-products, unreacted aromatic feed stock and conversion products derived from the aromatic feed stock. The condensed liquid is discharged from the settling drum 23S by Way of a bottoms discharge line 242 and may, if desired, be returned from thence to the reaction Zone 20d by Way of a recycle line 244 controlled by a valve 24o. fore preferably, however, the condensed liquids are processed in order to remove at least the Water and methanol lay-products from the system. This may be accomplished, in accordance with one procedure, by routing the condensed liquids by Way of a line 243 controlled by a valve 25d to a separation zone to be subsequently described. Y

Alternately, the condensed liquids are charged to a Q settling drum 252 by Way of a charge line 254i controlled by a valve 256 which branches from the line 24d.

Within the settling drum 252, the condensed liquids are separated into an aqueous phase and an oil phase containing principally components derived from the aromatic feed stock. rihe oil layer may be discharged from the drum 252 by the previously mentioned recycle line 218 leading to the aromatics charge line 2ll2.

The aqueous phase may be discharged by Way of a line 255i leading to a fractionation zone of any suitable f construction wherein water and methanol by-products may be removed.

By Way of example, the separation zone may comprise a first distillation column 2do and a second distillation column 262. The aqueous material is charged to the first distillation column by way of the line 253 and separated therein into a water fraction which is discharged from the bottom or the tower by Way of a bottoms discharge line 263 for discard from the system. An overhead product is taken from the rst distillation column 2&9 by Way of a line 251i leading to the second distillation Zone 262. Within the second distillation Zone 262, the charge material therefor is separated into a substantially anhydrous methanol bottoms fraction which is discharged by way or" the above-mentioned line 2tlg leading to the methanol charge line 2h54. Methanol by-products such as methylal and methyl formate are taken overheads by Way of a line 266 for-discard from the system.

Returning now to the reaction Zone 26d, a liquid product stream is continuously discharged by Way of a .product vline 263 leading to a ash drum 272. As a result of a reduction in pressure, water, methanol, methanol lay-products, and entrained oily material derived from the aromatic hydrocarbon feed stock are volatilized Whereas the principal portion of the unreacted feed stock' and conversion products thereof remains liquid. The oil layer is discharged from the flash drum 272 by a line 273 leading to a line 29d for further processing in a manner to be described.

The vapors from the flash drum 272 are taken overhead by way of a line 274 leading to a cooler 276 wherein substantially all of components of the vapor fraction are liquefied. The liquefied material is discharged from the cooler 276 by Way of a line 278 leading to a settler 28d. If desired, the valve 256 in the line 254 leading to the settling drum 272 may be closed and the valve 256 in the line 243 may be opened whereby condensed liquids obtained from the olf-gas may be added to the charge line 273 to the settler 28) by way of the previously mentioned line 248.

Within the settling drum 280, an oil phase and an aqueous phase collect. Off-gas, if any, is discharged from the settler 2S@ by Way of a vent line 282. The aqueous phase is withdrawn by way of a line 234 leading to a separation zone for the removal of Water and methanol lay-products. Suitably, the separation zone comprises a third distillation column 286 and a fourth distillation column 233. Within the third distillation column 285, the aqueous phase is separated into a bottoms Water fraction which is discharged by way of a bottoms line 2S9 for discard from the system. The lighter material, consisting principally of methanol and methanol Dy-products, is taken overhead by way of a line 29) leading to the fourth distillation column 283. In the fourth distillation column 235', a substantially anhydrous methanol bottoms fraction is obtained which is discharged by Way of a line 2M leading to the methanol charge line 264. Methanol by-products are taken overhead by way of a line 291i for discard from the system.

The oil fraction from the settler 280 is discharged by way of a line 292 containing the branch line 220 controlled by valve 222 which leads to the aromatics charge line 2112 and a branch line 294 controlled by a valve 296 leading to a separation zone 298. All or a selected portion of the oil introduced into the line 294 may be charged to the separation Zone 298. If desired, a selected fraction of the oil may be directly recycled to the reaction zone 2do' from line 29d by Way of the line 22) leading to the aromatics charge line 222.

Within the separation zone 298, a polymethyl ester of a polycarboxylic acid may be separated yfrom the products of the reaction by any suitable technique such as distillation, fractional crystallization, or a combination process involving both distillation and fractional crystallization. The thus-recovered product is discharged from the system by a products discharge line 30o and the remaining material is recycled to the line 22@ by Way of a line 262.

OPERATION By way of a specific example of a process conducted in the apparatus shown in FIG. 1, the charge material may be mesitylene and the desired product may be trimethyl trimesate. yIn lthis situation, a feed material `which may consist essentially of mesitylene is introduced by Way of the charge line 212 and is mixed in the inccrporatcr 2do with an amount of methanol suihcient to provide, by Way of example, about 6 mols of methanol per mol of mesitylene. An oxidation catalyst such as cobalt acetate and a promoter such as hydrogen bromide are mixed with the mesitylene by Way of `the catalyst charge line 2id. The resultant feed stream thatis formed in the incopporator 266 is passed through the preheater 226 and from thence to the reaction Zone 2do wherein the methyl groups on the mesitylene are oxidized ultimately to carboxyl groups and wherein the carboxyl groups are esteried with the methanol. it Will be understood that in a continuous process the oxidation catalyst and promoter will be added at a replacement rate and that fresh mesitylene will be added in an amount equivalent to the amount of trimethyl trimesate that is recovered in a manner to be described. Similarly, the fresh methanol is charged in an amount equivalent .to the amount consumed through esteriiication and degradation.

For the purpose of the present discussion, it may be assumed that the valves 246 and 255 in the lines 244 and 254, respectively, are closed. In this situation, off-gas from the reaction zone d is cooled in cooler 234 to condense liqueiiable material contained therein. Uncondensable o-gas is vented from the system by way of vent line 24d and the condensable material is charged by4 way of the line 248 to the line 278 leading to the settler 280.

The product stream is continually discharged from ythe zone 2th? by Way of theproduct discharge line 265 and ashed in the drum 272 to separate light components from the aromatic material present in the reaction product stream. The aromatic material will include a wide mixture of materials such as unreacted mesitylene, partial oxidation products thereof, partial esteriiication products, an'd fully oxidized fully esteried material which, in this instance, will be .trimethyl trimesate.

The gaseous material taken overhead by way of the line 274 from the zone 272 is -condensed in the cooler 276 and charged from thence by way of the line 273 to the settling drum 28d'. Residual oil components, if any, are discharged `from the settling drum 28d by way of the line 292 for yfurther processing and the aqueous phase is discharged by way of the line 284tfor separation in the distillation columns 286 and 28S into a Water fraction, an anhydrous methanol fraction, and a by-product fraction. Water is discarded from the system by way of Vthe bottoms discharge line 239 for the column 286 and byproducts are discarded from the system by way of the overheads line 290 leading from the column 233. The anhydrous methanol is recycled by way of the line 21%.

The aromatic material from the iiash drum 272 is discharged by way of the bottoms discharge line 273 and has admixed therewith aromatic material discharged from the settling drum 280` by way of the line 292.

Preferably, a major portion of the resultant mixture is directly recycled to the aromatics charge line l2 by way of the recycle line 220 controlled 4by the valve 222. A minor portion of the mixture is charged by way of the line 294 controlled by a valve 296 to the separation zone 293. In the separation zone 2%, purified trimethyl trimestate is recovered and discharged by way of a line 36d. Partial conversion products of the mesitylene are returned to the recycle line 22d by way of the line 302.

PLURAL STAGE REACTON head reactor lid and at least one tail reactor l2. The

reactors may suitably be provided with agitating means of any desired construction such as impellers 14 to 16, respectively.

A polyalkyl aromatic hydrocarbon feed stock such as paraxylene from a suitable source (not shown) is charged by way of a charge line i8 to a suitable mixing device such as a baflie plate incorporator 2t) and is there mixed with catalyst charged by way of the line 22, methanol charged by way of the line 2d controlled by a valve 26, and preferably a recycle fraction 28 obtained in a manner to be described. `As'indicated, in excess of l mol of methanol per methyl group to be oxidized is charged. Catalyst is charged by way of line 22 in a replacing amount in order to maintain the desired rate of reaction.

The reaction medium is discharged from the incorporator 2t) by way of a line 3d containing a preheater 32 leading to the reactor i0.

In this fashion, the mixture is charged to the reactor at a reaction temperature within. the range of about 140 to about 250 C. The pressure is adjusted so as to maintain the methanol substantially exclusively in liquid phase. Air or any other suitable source of ox gen is charged to the reactor by way of an air injection line 34 at such a rate that with the available gas-liquid contacting, an

on the aromatic hydrocarbon feed stock whereby such methyl groups may be ultimately converted to acid groups. Such acid groups, under the reaction conditions involved, are rapidly esterified with the methanol present. Only minimized side reactions of the methanol occur, Whereby not more than about 0.5 to l mol of alcohol is converted to other than ester products per mol equivalent of ester formed. Only a minor amount of the methanol is degraded to carbon dioxide and water.

When air in the oxidizing medium, as shown, the offgas containing entrained reaction components is discharged by way of a vent line 36 adjacent the top of the reactor liti and passed through a cooler 38 to condensate entrined or volatiiized normally liquid components contained therein. The remaining gas is vented from the system by way of a vent line 4h controlled by a valve d2 and the condensed liquids are returned to the system by way of a reflux line d4 controlled by a valve 46. The condensed liquids may include methanol, methanol degradation products, water, hydrocarbon feed, and oxidation products thereof.

Alternately, the valve 46 in the reflux line 44 is closed whereby the condensed liquids are charged by a branch line 48 controlled by a valve S0 to a settler 52 where the aqueous phase separates from the oil phase. ln this situation, the oil phase may be returned to the reflux line 4d by way of a return line 5d controlled by a valve Sri and the aqueous phase may be discharged by way of a line 5S controlled by a valve 59. Preferably the aqueous phase, rather than being discharged from the system, is charged by way of a branch line 60 controlled by a valve 6i to a fractionation zone to be described. In this situation, the amount of methanol withdrawn by way of the line 58 is continually replaced with an appropriate amount of methanol charged by way of the line 2.4 whereby the desired molar excess of methanol is maintained in the reactor iti.

The average residence time of the feed stock within the reactor il@ should preferably be sufficient to provide for the conversion of an average of at least one of the methyl groups of the xylene to an ester group. Thus, the residence time should preferably be such that about 6() to percent of the xylene feed stock is at least partially converted.

A product side stream is withdrawn from the. reactor )l0 by way of a discharge line 62 leading to a first fractionation zone 64. The first stage product 62 is preferably mixed with the recycle fraction dii described above and a recycle fraction 66 to be subsequently described prior to introduction into the fractionation zone 64. The fractionation zone 64 is represented in the drawing as a single distillation column. It will be understood, of course, that one or a plurality of distillation columns may be employed in the zone 64. Thus, the charge to the fractionation zone ad will be comprised of a mixture of unreacted feed stock, at least partially reacted feed stock, methanol, methanol degradation products, water, catalyst components, etc. Within the fractionation zone 64, the mixture is split into a light fraction 65 containing methanol, water, and, in the case of incomplete fractionation, entrained oily materials. The light fraction 68 is cooled in a cooling zone 7@ and charged to a separator 72 wherein any entrained hydrocarbon materials are separated from an aqueous methanol solution by phase separation. The oily phase is discharged by way of a line V"7d controlled by a valve 76 and the aqueous phase is discharged by way of a line 7 8.

The aqueous phase is charged to a second fractionation zone comprising, for example, a first distillation column di) and a second distillation column 82. The aqueous 9 fraction 78 is initially charged to the column 80 wherein a hght fraction composed principally of methanol oxidatslon lby-products removed overhead by way of the line The bottoms fraction consisting essentially of methanol, water, and entrained catalyst components is discharged by way of a line 84 leading to the second distillation column 82. Within the second column 82, the aqueous medium is separated into a substantially anhydrous methanol fraction d6 which will also contain entrained catalyst components. The methanol fraction 86 may be discharged from the system but preferably is recycled by way of a recycle line $8 controlled by a valve 9d. Thus, all or a portion of the methanol recovered by way of the line S6 may be recycled to the methanol charge line 24 leading to the incorporator 20.

Returning now to the iirst fractionation zone 64, a heavier fraction composed principally of unreacted xylene and at least partial conversion products thereof is discharged by way of a line 92 and, in admixture with entrained oil charged ythereto by =way of the line 74, is charged to a second incorporator 94 wherein the material is mixed with -an amount of methanol at least sufficient to replace that withdrawn overhead from the zone 6d by way of the line 68. The methanol may be charged to the incorporator 9d by way of a charge line 96 controlled by a valve 98. All or a desired portion of the methanol may be recycle methanol introduced into the line 96 by way of a branch line ltltl controlled by a valve 102 interconnecting the methanol charge line 916 with the alcohol recycle line 88.

The thus-reconstituted partially converted reaction mixture is discharged from incorporator 94 by Way of a line lil/i containing a preheater lilo leading to the second reactor 12. The charge mixture for the second Areactor l2 may be heated to a temperature Within the range of 140 to 250 C. which is the same as or different from the temperature employed in the reactor lll. Air or other suitable oxygen-containing gas is charged to the reactor l2 by way of a charge line idg at a rate such that, with the available gas-liquid contacting, an oxygen utilization of about 50 percent or greater is obtained.

Within the reactor l2, additional conversion of the hydrocarbon charge material occurs including further oxidation of methyl groups and further esterication of carboxyl groups. Ofi-gas is dichrarged by way of a vent line llltl leading to a cooler M2 wherein condensable liquids including water, methanol, methanol by-products, and aromatic components are condensed. Uncondensable gases consisting principally of nitrogen are discharged from the cooler by way of a line llt and the condensed material is discharged by way of a reflux line lid controlled by a valve 118.

As before, the condensed liquids may alternately be charged by Way of a branch line l2@ controlled by a valve lt22 leading to a separator 124 wherein ,an aqeous phase and an oil phase are formed. The aqueous phase may be discharged from the system by way of a discharge line 126 controlled by a valve t28. Preferably, the aqueous phase is recycled by way of a branch line i3d controlled by a valve H2 leading to the charge line 66 tor the fractionation zone 64 ln this situation, the oil phase is returned to the reactor l2 by way of a return line 134 controlled by a valve i3d and an amount of methanol is added by way of the methanol charge line 9d sufiicient to replace methanol withdrawn by way of the line 130.

A stream of oxidation products is withdrawn from the reactor l2 by way of a discharge line 13S lea-ding to a third fractionation zone 140.

Within the Zone Mtl, the product is separated into a lighter fraction composed of conversion products having a boiling point less than the boiling point of a desired esteried aromatic product. A bottoms fraction lis discharged from the zone Mtl by way of a line 142 leading to a second fractionation zone 14M- wherein a desired product is obtained overhead by way of a line 146 and wherein a heavier fraction is discharged by way of a -bottoms line Mii. All or a selected portion of the bottoms fraction it-8 may be discharged from the system by way of a branch line ltl controlled by a valve 152. Alternately, the heavier fraction may be recycled to the initial incorporator Ztl by way of the recycle line 2d.

The product fraction discharged from the zone 1144 by way of the line M6 may consist of fully esteriiied polycarboxylic aromatic acid (e.g., dimethyl terephthalate). Alternately, the fractionation conditions may be adjusted so that the product ystream ldd also includes partially esteried aromatic polycarboxylic acid components. In this situation, the incompletely esteritied material may be charged by way of a branch line T154 controlled by a valve ld to an incorporator i555 wherein the material may be mixed with an excess of methanol charged by way of a line ldd controlled by a valve i162.. A suitable esteritication catalyst such as a mineral acid may be added by lway of a charge line llo-f!- controlled by a valve idd.

The mixture may be discharged from the incorporator '158 by way of a line 168 containing, if necessary, a preheater 169 and leading to an esteriiication Zone 170 containing suitable agitating means such as an impeller 172. Within the esteriflcation zone 170, the esterifrcation of the aromatic carboxylic acid material is driven to completion under suitable esterication conditions. A product stream may be Withdrawn from the esterication Zone l'tl by way of a line l74 leading to a separation zone 176 wherein the product stream may be separated into -a methanol phase and an ester phase. The ester phase, consisting essentially of fully esteried aromatic carboxylic acid, is discharged by way of ya discharge line 178 and the methanol phase is recycled (eg, to the methanol charge line lr6@ by way of -a recycle line 179). In order -to prevent an undue build-up of water in the system, at least a portion of the methanol phase may be delivered by way of a branch line lltl controlled by a valve 182 to the charge line 84 for the yfractionation Zone 82.

OPERATION By way of a specific example of the manner in which a dimethyl terephthalate may be obtained `from paraxylene, the -following sequence may be followed.

At least about 3 mols of methanol per mol of paraxylene in the mixing Zone Ztl and a suitable amount of oxidation catalyst such as cobalt naphthenate. Hydrogen bromide is also added.

The thus-prepared reaction mixture is charged to the reactor lil wherein at least about percent of the paraxylene is converted to oxidation products. That is to say, an average of about l of the methyl groups per molecule is at least partially oxidized and Iat least a substan-tial portion of the carboxyl groups that `are formed are esteriiied.

A product stream is withdrawn `from the reactor lt) by way of the line 62. The product stream will contain oxidation products of paraxylene, including dimethyl terephthalate, methyl paratolua-te, monomethyl terephthalate, methyl benzoate, paratolualdehyde, paratoluic acid, paraxylyl alcohol, nnreacted paraxylene, and also methylal, methylacetate, methyl formate, methanol `and water.

The mixture is fractionated in the described manner to provide an essentially `aqueous phase which is discharged overhead by way of the line 63 from lthe fractionation Zone 6d `and a heavy essentially oil phase discharged by way of the bottoms line 92. The oil phase 92 will consist essentially of unconverted paraxylene and the oxidation-esteriiication products thereof.

The heavy phase 92 is charged to the incorporator 94 wherein it is mixed with an amount of methanol suicient to provide at least about 3 mols of methanol per mol of initial paraxylene charge stock. The resutlant mixture is charged t`o the second reactor 12 -by way of the line 104 wherein the production of dimethyl terephthalate and monomethyl terephthalate is maximized. The product stream 138 withdrawn from the reactor 12 will contain essentially the same components as were charged thereto but in different amounts.

The product stream 138 is charged to a Zone 140 whereinan overheads `fraction 66 is obtained cotnaining substantially all of the methanol and byproducts thereof. A heavy phase containing paraxylene oxidation products is withdrawn from the zone 140 by way of a line 142 leading to a fractionation zone 144 wherein an overheads product `fraction 146 is obtained and wherein a bottoms recycle fraction 148 is obtained which may be recycled by way of the line 28.

The overheads fraction 145 may consist of dimethyl terephthalate, in which event this will be the nal product. Alternately, the overheads fraction 14a, may consist of a mixture of dimethyl terephthalate with monomethyl terephthalate. In this situation, the mixture may be charged by way of the branch line 154 to the incorporator 158 wherein an additional substantial excess of methanol may be added, together with an acidic esterification catalyst such as a mineral acid. The resultant mixture is charged to the esterication Zone 170 wherein esterication is driven to completion. The esteriiication product is `discharged by way o-f -a line 174 leading to a separation zone 176.

A product fraction consisting essentially of dimethyl terephthalate is discharged by way of a line 178 and a methanol fraction is discharged by way of a discharge recycle line 179.

The invention will be further illustrated by Way of the yfollowing specific examples which are `given by Way of illustration and not as limitations on the scope of this invention.

Example I yA mixture of methanol and orthoxylene was prepared, the mixture being proportioned so as to contain about 3 mols of methanol per mol of orthoxylene. To this mixture there was added about 2 percent cobalt acetate, about 2 percent manganese acetate and about 2 percent hydrogen bromide (based on the weight percent of the mixture). The resultant mixture was then subjected to a 2-stage oxidation-esteritication reaction. Reaction conditions in the first stage included a temperature of about 360 =E. (about 180 C.), a pressure of about 550 p.s.i.g., and a flow rate of about 1 volume of total feed per reactor volume per hour. Air was flowed through the reactor at a rate to provide maximum oxygen utilization, in this case at the rate of about moles of air per mol of orthoxylene per hour. The composition of the total reactor product from the irst stage is set forth in Table i.

TABLE L-PRODUCT DISTRIBUTION, STAGE I Product component: Weight percent Dimethyl phthalate 3.4 yMonomethyl phthalate 10.9

Methyl benzoate 1.3 Methyl orthotoluate 2.8 `Orthotolualdehyde 4.5 Orthotoluic acid 18.7 Phthalide 9.4 Orthoxylene 4.4 Orthoxylyl alcohol 1.2 Methylal 2.8 `Methyl acetate 1.4 Methyl formate 0.5 Methanol 20.3

Water 18.4

The total product from stage 1 was fractionated in order 12 tion products thereof, was charged to the second stage, together with an added amount of methanol suicient to provide about 3 mols of methanol per mol of xylene-type compounds. Since the catalyst components initially charged to the first reaction zone were present in the oil phase, no additional catalyst was added.

The second stage reaction conditions included a temperature of about 390 F. (about 200 C), a pressure of about 700 p.s.i.g., a ilow rate of about 0.5 volume of total feed per reactor volume per hour, and an air charge rate of about 15 mols of air per mol of xylene-type compounds per hour. The total product from the second stage was recovered and analyzed and found to have the composition set forth in Table II.

to separate methanol, water, methyl formate, methyl acei It will be observed from Table II that there was a high yield and selectivity to dimethyl phthalate and monomethyl phthalate.

From the foregoing Tables I and Il, it will be further observed that phthalide formed in the rst stage is partially converted in the second stage. Consequently, loss of orthoxylene through the formation of by-product phthalide may be avoided by recycle of the phthalide inasmuch as the phthalide is converted under the process of the present invention to dimethyl phthalate.

It is to be noted that all of the agents necessary for the process were added in the beginning in this experiment. That is to say, both the heavy metal catalyst and :the hydrogen bromide were added to the rst stage. Replenishment was not required for the second stage reaction.

Thus, when addi-tional hydrogen bromide was added dur- In order to illustrate the need `for maintaining the methanol in liquid phase as much as possible, a comparison was made with respect to the results set forth in Example I wherein methanol converted to by-products amounted to about 2 mols per mol of dicarboxylic acid yield and the results of the experiment described below.

The test run which was made for comparative purposes was a batch run with respect to the orthoxylene to be converted in that all of the xylene was charged at the beginning of the run. However, the run was semi-continuous with respect to methanol. The charge stock for the experiment was a mixture of methanol with orthoxylene proportioned so as to provide for about 4 mols of methanol per mol of orthoxylene. In addition, about 2 weight percent of cobalt acetate, about 2 weight percent of manganese acetate and about 2 weight percent of hydrogen bromide were added. The above weight percentages are calculated as weight percentages based upon the weight of the mixture. rthe reaction was conducted at a temperature of about' 380 F. and air was charged at the rate of about 10 mols of air per mol of orthoxylene charge per hour. YIn this instance, however, the reaction pressure was lowered to about 450 p.s.i.g. whereby a relatively large amount of condensate was obtained 4from the off-gas. This was attributable to the fact that the reaction Was conducted in this instance very near the boiling point of methanol and Water. The condensate which contained both water and methanol was not returned to the reactor, but an equivalent amount of dry methanol was continually supplied during the course of the reaction. In this manner, the water concentration in the reactor was minimized. However, a high throughput of methanol in the vapor phase was necessary in Order to accomplish this result. After about 4 hours of operation, the condensate was collected and analyzed to determine the amount of methanol converted to byproducts and the products of the reactor were analyzed to determine the xylene conversion. It was found that a yield of about 70 percent of dicarboxylic material had been obtained. However, more than about 9 mols of methanol were converted to by-products per mol of dicarboxylic product.

Thus, under comparable conditions of temperature, concentration, residence time, and yield of dicarboxylic material, but with large amounts of methanol in the vapor phase and a low concentration of Water in the reactor, about 5 times as much methanol was converted to by-products as compared with Example I.

Example III A mixture of methanol with mesitylene in the ratio of about 6 mols of methanol per mol of mesitylene was prepared. To the mixture was added about 2 Weight percent each lof Co(OAc)2-4H2O and Mn(OLAc)2-4H2O plus about 2 percent of hydrogen bromide. The resultant mixture Was reacted for about 6` hours under reaction conditions including a temperature of about 185 C., a pressure of about 500` p.s.i.g., and a gas ow rate of about 9 mols of air per mol of mesitylene charge per hour.

Since a complete analytical procedure was not available for analyzing the products from the above example, the following procedure was employed. Light materials such as methanol and water were evaporated from the product. Fresh methanol was added to the product (6 mols of methanol per mol of feed) in order to convert all of the acid components of the product to esters. This was done in order to simplify the analysis. The esteriiication reaction was conducted at a temperature of about 160 F. with anhydrous hydrogen chloride being continually added as catalyst. At the end of about 4 hours, the esteriflcation mixture Was cooled to room temperature and a substantial amount of crystalline material precipitated. This material was collected and dried and analyzed. It was found to consist of essentially pure trimethyl trimesate.

Example IV To a mixture of p-diisopropyl benzene with methanol in the ratio of about 4 mols of methanol per mol of diisoprpoyl benzene there was added about 2 weight percent each of Co(OAc)24H2O and Mn(OAc)2-4H2O. The resultant reaction mixture was reacted for `about 5 hours under reaction conditions including a temperature of about 175 C., a pressure of about 500 p.s.i.g., and an air flow rate of about 9 mols of air per hour per mol of diisoprpoyl benzene. A substantially constant volume of liquid material was maintained in the reactor by replacing condensable materials carried with the oit-gas as vapors with equivalent amounts of methanol. As a 14 consequence, oxidation of the p-diisopropyl benzene was initiated and esteritication of carboxyl groups formed by the oxidation was initiated. After about 1 hour of reaction, aqueous hydrogen bromide (48 percent solution) Was introduced in order to promote the oxidation and esterication reactions.

Since a complete analytical procedure was not available for analyzing the products from the above reaction, the following procedure was employed. Methanol and water were removed from the reaction lmixture and the remaining oil solids were slurried in boiling methanol and filtered hot. On cooling, the crystals that precipitated from the filtrate were recovered and were dissolved in hot methanol. On cooling, a crystalline product was obtained which was found by analysis to consist essentially of pure dimethyl terephthalate.

What is claimed is:

l. In a method of preparing a product aromatic compound containing at least one methyl ester group by reacting in the liquid phase a polya-lkyl aromatic hydrocarbon having from 1 to 4 carbon atoms in each alkyl substituent with methanol and molecular oxygen in a reaction zone in the presence of a catalyticall-y effective amount of a polyvalent heavy metal containing oxidation catalyst, the improvement which comprises conducting the reaction at a temperature and pressure correlated to maintain said methanol preferentially in the liquid phase, whereby oxidative degradation of methanol is minimized.

2. A method in accordance with claim 1 wherein the polyalkyl aromatic hydrocarbon is a Xylene.

3. In a method of preparing a product aromatic compound containing at least one methyl ester group by reacting in the liquid phase a polyalkyl aromatic hydrocarbon having from 1 t0 4 carbon atoms in each alkyl substituent with methanol and molecular oxygen in the presence of a catalytically effective amount of a polyvalent heavy metal containing oxidation catalyst at a temperature within the range of about 140 C. to about 250 C. and at a pressure within the range of about 200 p.s.i.g. to about 1000 p.s.i.g., the improvement which comprises conducting the reaction at a temperature and pressure correlated to maintain said methanol preferentially in the liquid phase and in a concentration within the range of from about 15% to about 50%, whereby oxidative degradation of methanol is minimized.

4. A method in accordance with claim 3 wherein the polyalkyl aromatic hydrocarbon is a xylene.

5. In a method of preparing dimethyl terephthalate by reacting in the liquid phase p-xylene with methanol and molecular oxygen in the presence of a catalytically effective amount of a polyvalent heavy metal containing oxidation catalyst at a temperature within the range of about C. to about 250 C. and at a pressure within the range of about 200 p.s.i.g. to about 1000 p.s.i.g., the improvement which comprises conducting the reaction at a temperature and pressure correlated to maintain said methanol preferentially in the liquid phase and in a concentration within the range of from about 15% to about 50%, whereby oxidative degradation of methanol is minimized.

6. A method in accordance with claim 5 wherein the catalyst is a `soluble salt of cobalt.

References Cited in the file of this patent UNITED STATES PATENTS 2,415,800 Rust et al Feb. l1, 1947 2,833,816 Safer et a-l May 6, 1958 2,879,289 Johnson Mar. 24, 1959 

1. IN A METHOD OF PREPARING A PRODUCT AROMATIC COMPOUND CONTAINING AT LEAST ONE METHYL ESTER GROUP BY REACTING IN THE LIQUID PHASE A POLYALKYL AROMATIC HYDROCARBON HAVING FROM 1 TO 4 CARBON ATOMS IN EACH ALKYL SUBTITUTING WITH METHANOL AND MOLECULAR OXYGEN IN A REACTION ZONE IN THE PRESENCE OF A CATALYTICALLY EFFECTIVE AMOUNT OF A POLYVALENT HEAVY METAL CONTAINING OXIDATION CATALYST, THE IMPROVEMENT WHICH COMPRISES CONDUCTING THE REACTION AT A TEMPERATURE AND PRESSURE CORRELATED TO MAINTAIN SAID METHANOL PREFERENTIALLY IN THE LIQUID PHASE, WHEREBY OXIDATIVE DEGRATION OF METHANOL IS MINIMIZED. 